Adhesive compositions based on styrenic block copolymers as thermoplastic elastomer components are well known in the art. These compositions are for instance used as PSA (pressure sensitive adhesive) for industrial tapes, packaging tapes and labels, and in multipurpose hot-melt adhesive compositions which may be used to bond or construct articles in the manufacture of disposable soft goods, such as diapers, feminine care articles, surgical drapes and the like.
Styrene-isoprene-styrene block copolymers (‘S-I-S’) and styrene-butadiene-styrene block copolymers (‘S-B-S’) are widely used in these adhesive compositions. Both classes of block copolymers give the adhesive compositions specific properties related to the respective inherent characteristics of these block copolymers. For example, the softness of S-I-S makes this polymer the material of choice for pressure sensitive applications in tapes and labels. Alternatively, the elevated cohesion of S-B-S makes this material attractive for construction adhesives for disposable soft goods.
When compounded into hot melt adhesives S-I-S polymers degrade by a chain scission mechanism; molecular weight is reduced and the cohesive strength of the adhesive is lowered. S-B-S polymers on the other hand tend to degrade by further chemical cross-linking increasing the cohesive strength of the adhesive, but also increasing the elastic modulus, forming a too hard and non-tacky adhesive. The thermal decomposition of both S-I-S and S-B-S based adhesives can ruin the utility of the adhesive product. It would be an advantage to the hot melt adhesive industry if polymers with less tendency to either fall apart (scission) or cross-link would be developed. In EP-669350 and U.S. Pat. No. 5,583,182, adhesive compositions have been described wherein the styrenic block copolymer is a block copolymer of an S-B-I-S type, an (S-B-I)n-X type or an (S-I-B)n-X type, wherein S represents a polystyrene block, B represents a polybutadiene block and I represents an isoprene block. These copolymers with block copolymer midblocks “B-I” combine some of the characteristics of S-I-S and S-B-S type polymers. However, the process to make blocks of polydienes in the midblock is demanding, more complex and requires long polymerisation times. It has been also demonstrated that because of the blocky structure in the midblock, their heat stability performance is still inadequate and that the hot-melt viscosity of formulated adhesives is too elevated for industrial applications compared to respectively S-I-S or S-B-S based formulations.
In WO00/14170 an adhesive composition is described based on an elastomeric component comprising (I) an SIS block copolymer and (II) an SBS block copolymer, and on a tackifying component comprising (III) a first hydrocarbon resin compatible with said SIS block copolymer and (IV) a second hydrocarbon resin compatible with said SBS block copolymer. However, the necessity to use two block copolymers (SIS and SBS) as well as two hydrocarbon resins is not a cost effective approach for the manufacture of adhesive compositions, as it requires the adhesive manufacturers to invest deeply in expensive feeding and dosing systems on their extruders.
In DE-2942128 an adhesive composition is described based on 100 part of a non hydrogenated block copolymer A-B-A where A is polystyrene and B a block made of a mixture of Butadiene and Isoprene; from 25 to 300 part of a tackifier resin; from 5 to 200 part of a plasticiser, and some additional additives. Actually, the tackifier resin is a mixture of resins, as is illustrated in the example (50 parts of a glycerine rosin ester (“FLORAL” 85, a trademark) and 50 parts of a synthetic polyterpene resin (“WINGTACK” 95, a trademark)). This mixture appears to be required to be reasonably compatible with the base adhesive polymer.
Mixtures of S-B-S and S-I-S type copolymers, as suggested in the art, do not provide an acceptable alternative either. There is therefore still a need for a pressure sensitive adhesive composition that is at least equal to those based on S-I-S, and indeed superior in heat stability to those based on S-I-S, S-B-S and/or S-I-B-S type block copolymers, and may be based on a single hydrocarbon resin as tackifier resin.